A race is a population of humans distinguished from other populations. The most widely used racial categories are based on visible traits (especially skin color and facial features). Conceptions of race, as well as specific racial groupings, vary by culture and time and are often controversial due to their impact on social identity hence identity politics.

Since the 1940s, evolutionary scientists have rejected the view of race according to which a number of finite lists of essential characteristics could be used to determine a like number of races. By the 1960s, data and models from population genetics called into question taxonomic understandings of race, and many have turned from conceptualizing and analyzing human variation in terms of race to doing so in terms of populations and clines instead. However, many scientists believe that race is a valid and useful concept. Moreover, since the 1990s, data and models from genomics and cladistics have resulted in a revolution in our understanding of human evolution, which has led some to propose a new "lineage" definition of race. These scientists have made related arguments that races are valid when understood as fuzzy sets, clusters, or extended families. Currently, opinions differ substantially within and among academic disciplines.

Many evolutionary and social scientists, drawing on such biological research, think common race definitions, or any race definitions pertaining to humans, lack taxonomic rigour and validity. They argue that race definitions are imprecise, arbitrary, derived from custom, and that the races observed vary according to the culture examined. They further maintain that race is best understood as a social construct. Other scientists, however, have argued that this shift is motivated more by political than scientific reasons.

At the beginning of the 20th century, anthropologists questioned, and subsequently abandoned, the claim that biologically distinct races are isomorphic with distinct linguistic, cultural, and social groups. Then, the rise of population genetics led some mainstream evolutionary scientists in anthropology and biology to question the very validity of race as scientific concept describing an objectively real phenomenon. Those who came to reject the validity of the concept, race, did so for four reasons: empirical, definitional, the availability of alternative concepts, and ethical..

The first to challenge the concept of race on empirical grounds were anthropologists Franz Boas, who demonstrated phenotypic plasticity due to environmental factors, and Ashley Montagu, who relied on evidence from genetics. Zoologists Edward O. Wilson and W. Brown then challenged the concept from the perspective of general animal systematics, and further rejected the claim that "races" were equivalent to "subspecies".

One of the crucial innovations in reconceptualizing genotypic and phenotypic variation was anthropologist C. Loring Brace's observation that such variations, insofar as they are affected by natural selection, migration, or genetic drift, are distributed along geographic gradations; these gradations are called "clines". This point called attention to a problem common to phenotypic-based descriptions of races (for example, those based on hair texture and skin color): they ignore a host of other similarities and difference (for example, blood type) that do not correlate highly with the markers for race. Thus, anthropologist Frank Livingstone's conclusion that, since clines cross racial boundaries, "there are no races, only clines". In 1964, biologists Paul Ehrlich and Holm pointed out cases where two or more clines are distributed discordantly—for example, melanin is distributed in a decreasing pattern from the equator north and south; frequencies for the haplotype for beta-S hemoglobin, on the other hand, radiate out of specific geographical points in Africa. As anthropologists Leonard Lieberman and Fatimah Linda Jackson observe, "Discordant patterns of heterogeneity falsify any description of a population as if it were genotypically or even phenotypically homogeneous".

Finally, geneticist Richard Lewontin, observing that 85 percent of human variation occurs within populations, and not between populations, argued that neither "race" nor "subspecies" was an appropriate or useful way to describe populations. This view is purportedly debunked as Lewontin's Fallacy. Some researchers report the variation between racial groups (measured by Sewall Wright's population structure statistic FST) accounts for as little as 5% of human genetic variation. However, because of technical limitations of FST, many geneticists now believe that low FST values do not invalidate the suggestion that there might be different human races. Meanwhile, neo-Marxists such as David Harvey believe that race is a social construct that in reality does not exist, used instead to extenuate class differences.

These empirical challenges to the concept of race forced evolutionary sciences to reconsider their definition of race. Mid-century, anthropologist William Boyd defined race as:

A population which differs significantly from other populations in regard to the frequency of one or more of the genes it possesses. It is an arbitrary matter which, and how many, gene loci we choose to consider as a significant "constellation".

Lieberman and Jackson have pointed out that "the weakness of this statement is that if one gene can distinguish races then the number of races is as numerous as the number of human couples reproducing." Moreover, anthropologist Stephen Molnar has suggested that the discordance of clines inevitably results in a multiplication of races that renders the concept itself useless.

Alongside empirical and conceptual problems with "race" following the Second World War, evolutionary and social scientists were acutely aware of how beliefs about race had been used to justify discrimination, apartheid, slavery, and genocide. This questioning gained momentum in the 1960s during the U.S. civil rights movement and the emergence of numerous anti-colonial movements worldwide.

In the face of these issues, some evolutionary scientists have simply abandoned the concept of race in favor of "population." What distinguishes population from previous groupings of humans by race is that it refers to a breeding population (essential to genetic calculations) and not to a biological taxon. Other evolutionary scientists have abandoned the concept of race in favor of cline (meaning, how the frequency of a trait changes along a geographic gradient). The concepts of population and cline are not, however, mutually exclusive and both are used by many evolutionary scientists.

In the face of this rejection of race by evolutionary scientists, many social scientists have replaced the word race with the word "ethnicity" to refer to self-identifying groups based on beliefs in shared religion, nationality, or race. Moreover, they understood these shared beliefs to mean that religion, nationality, and race itself are social constructs and have no objective basis in the supernatural or natural realm.